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Length Contraction


Deepak Kapur

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A spaceship in the sky whizzes past an observer at a tremendous speed.

 

The observer sees and measures its lenght to be 10 metres. The people on the spaceship measure it to be 12 metres.

 

The same spaceship again whizzes past the observer at the same speed.

 

This time, the observer uses his ultra powerful telescope to view the space ship so that he can have a very close view.

 

What would be the length of the spaceship as seen and measured by the observer? 10m or 12m?

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@ strange

 

 

 

Does it mean the space in the space ship also contracts?

 

Does it mean there is no real length?

 

Does it mean from the view point of particles in the LHC, we are just point entities?

 

..........................all this seems philosophy, not science:/

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Does it mean there is no real length?

The length does depend on who measures it. However, there is the notion of proper length which is the length as measured in a co-moving inertial frame. The word proper may not be the best choice here, but is is what is now standard.

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@ ajb

 

( I am replying from mobile....somehow cannot quote)

 

IMO here the words proper, standard or other such variants are misnomers at best......

 

BTW, when the observer focuses on the space ship, doesn't he become one-to-one with the reference frame of the space ship????

@ everybody

 

What is the mechanism through which speed and length interact with each other, so as to influence each other......no maths please...a dumber is at the receiving end.

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@ swansont

 

plz be elaborate (what you said is obvious)

 

So, does it mean atoms/forces rearrange themseves in the spaceship for the observer who is not in the spaceship?

 

Who/what tells the atoms to get arranged in a new fashion?

 

if the atoms don't get rearranged,can this phenomenon be called a special type of optical illusion??

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@ swansont

 

plz be elaborate (what you said is obvious)

It's not obvious to me what's not obvious to you. You asked if an observer "become(s) one-to-one with the reference frame of the space ship". The answer is no. The measurement depends on your frame.

 

So, does it mean atoms/forces rearrange themseves in the spaceship for the observer who is not in the spaceship?

 

Who/what tells the atoms to get arranged in a new fashion?

 

if the atoms don't get rearranged,can this phenomenon be called a special type of optical illusion??

 

The atoms do not rearrange themselves. What their "arrangement" (separation) depends on the frame used to observe them, but observation is not an interaction that causes any physical effect. The length we measure is frame-dependent rather than being an invariant.

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if the atoms don't get rearranged,can this phenomenon be called a special type of optical illusion??

 

It is similar to, say, perspective in that it is caused by geometry. But I think it is misleading to call it an illusion; the different measurements made by each frame of reference are both equally real (without getting into a philosophical discussion of what "real" means).

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wouldn't the length be the same 12 m in all cases, only appearing at 10m from the distortion due to the stationary observer and the observed moving relative to one another so quickly? I presume the people in the spacecraft would see a similarly distorted stationary observer, tall and squashed front to back should they look out the window...

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(Very simply ...) When it turned out that the speed of light is constant, it meant that distance and time must be the things that are variable.

 

A person in the spaceship must observe light to move at the speed of light relative to them (e.g. crew member A at the bow flashes a light at crew member B at the stern). The outside observer C must also observe that same light to move at the speed of light relative to them ( C ), even though the spaceship itself is also moving relative to them ( C ).

 

For them all to conclude that the light they measured moves at the speed of light, distance and time must be varying. (i.e. one of them must measure the light to move a shorter distance than the other does, and in less time than the other).

 

(Edit: Also, from the point of view of C, B is moving towards the light source, but from the point of view of A and B, they are at rest and the Universe is moving; it's complicated.)

 

This isn't an optical illusion.

Edited by pzkpfw
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There's no experiment you can do that would show that 12m is right and 10m (or whatever the length is is in any other frame) isn't. If I'm in a frame where I measure 10m, all of the physics I do where that length is important will only work if 10m is the length.

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What is the mechanism for this to happen- rather than saying this just happens or pointing to the equations....

 

Does this happen instantaneously or is subject to c?

 

Suppose, a spaceship is travelling at tremendous speed near the outer edge of the universe and the light from it reaches us after billions of years, so when does this EFFECT of relativity come into existence....now or billions of years earlier....

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What is the mechanism for this to happen- rather than saying this just happens or pointing to the equations....

 

Does this happen instantaneously or is subject to c?

 

Suppose, a spaceship is travelling at tremendous speed near the outer edge of the universe and the light from it reaches us after billions of years, so when does this EFFECT of relativity come into existence....now or billions of years earlier....

There is no mechanism, per se, as there is no physical change going on. What you see/measure is a function of your relative speed with respect to the target. It is a ramification of the speed of light being finite and invariant, and our choice of how we have defined things like simultaneity and clock synchronization, and the idea that physics has to work in all frames of reference.

 

You are free to try and think up a set of rules where length is an invariant quantity, but it will cause serious complications elsewhere in your physics.

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1. How does length know it has to contract and how does time know it has to dialate?

 

2. Will this happen if no observer is present?

 

3. When a photon is produced, what tells it to move at c?

 

4. How does the universe know it has to maintain the constancy of speed of light?

 

5. How does universe know that an observation is taking place, so that relativity effects are to actualize?

 

i am not much intelligent, but somehow a thought has come into my mind that on this forum certain questions are ignored/not commented upon ( this applies to threads/comments started by others also, some of whom are long standing members)

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1. How does length know it has to contract and how does time know it has to dialate?

 

It obviously doesn't know any such thing. One person makes a measurement and gets one result; another person makes a measurement and gets another result.

 

If these two observers measured the speed of the object (relative to them), they would both get different results but that doesn't seem surprising.

 

 

3. When a photon is produced, what tells it to move at c?

 

4. How does the universe know it has to maintain the constancy of speed of light?

 

Who knows. That just seems to be the way the universe is built. Maybe it is impossible for a universe to behave otherwise.

 

 

5. How does universe know that an observation is taking place, so that relativity effects are to actualize?

 

I'm not sure that makes sense. It is just about measurements made from different frames of reference.

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1. How does length know it has to contract and how does time know it has to dialate?

 

2. Will this happen if no observer is present?

 

3. When a photon is produced, what tells it to move at c?

 

4. How does the universe know it has to maintain the constancy of speed of light?

 

5. How does universe know that an observation is taking place, so that relativity effects are to actualize?

 

i am not much intelligent, but somehow a thought has come into my mind that on this forum certain questions are ignored/not commented upon ( this applies to threads/comments started by others also, some of whom are long standing members)

1. Time and length don't "know" anything. They aren't endowed with knowledge.

2. The laws of nature don't care about our presence or absence.

3. No one knows, this is why it is an axiom.

4. Same answer as 3.

5. Same answer as 1.

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1. How does length know it has to contract and how does time know it has to dialate?

 

2. Will this happen if no observer is present?

 

3. When a photon is produced, what tells it to move at c?

 

4. How does the universe know it has to maintain the constancy of speed of light?

 

5. How does universe know that an observation is taking place, so that relativity effects are to actualize?

 

i am not much intelligent, but somehow a thought has come into my mind that on this forum certain questions are ignored/not commented upon ( this applies to threads/comments started by others also, some of whom are long standing members)

Pick up a dinner plate and look at it straight on. You see a circle. Now turn the plate so that you are looking at it at an angle. You see an oval. How does the plate "know" that it is supposed to "change shape" when looked at from different directions? The answer is that there is nothing that "tells" the plate how it should look. It's appearance is just a result of what angle it is looked at from.

 

Asking how an object knows that it must contract or that its time must dilate is like asking how the plate knows how to look different when viewed from a different angle. It is a meaningless question. Length contraction and time dilation are not due to anything acting on a "moving" object, it is due to the fact that frames in relative motion measure time and space differently from each other, just like the plate looks different when viewed from different directions.

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@ Janus

 

You gave the example of different views of a plate...if someone derives an equation for these views, would you call that a law of the universe, a relativity law?

 

So, why say something same for relativity when basically it tells of different views of a plate called universe....

@ Janus

 

You gave the example of different views of a plate...if someone derives an equation for these views, would you call that a law of the universe, a relativity law?

 

So, why say something same for relativity when basically it tells of different views of a plate called universe....

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@ Janus

 

You gave the example of different views of a plate...if someone derives an equation for these views, would you call that a law of the universe, a relativity law?

 

So, why say something same for relativity when basically it tells of different views of a plate called universe....

@ Janus

 

You gave the example of different views of a plate...if someone derives an equation for these views, would you call that a law of the universe, a relativity law?

 

So, why say something same for relativity when basically it tells of different views of a plate called universe....

 

In very rough terms, yes. Relativity gives you the equations that work for your view of the universe. There no "correct" reference frame from which views the universe — the laws of physics work in all inertial frames. This means there is no absolute speed.

 

Using he plate example, you could find the equation for what the plate looks like from your frame; the projection of a circle that depends on the angle. The plate doesn't change, and does need to "know" anything in order to look like an oval.

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@ swansont

 

you said the plate doesn't change.....

 

IMO there is no absolute plate to talk about, just like there is no absolute speed to talk about....a plate to me can be a dot to somebody else...

 

moreover, when the main stays of our universe ( space, time) are not absolute....shouldn't sth absolute be the aim of science, rather than saying that science has no absolute aim...

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